Electronic devices capable of digitizing and reproducing information manually entered upon an electrically communicative surface are relatively new to the field of information processing. Previous devices of this type fall generally into one of two categories; digitizing tablet systems and electronic copyboards, both of which possess significant technological limitations as compared to the abilities provided by the electronic touch sensitive position sensor of the present invention.
Digitizing tablet systems of the prior art generally include an electrically conductive, X-Y grid like structure of wires underlying an exposed data entry and display surface associated therewith and a stylus mechanism which is coupled to the grid structure. The stylus mechanism may be inductively, resistively or capacitively coupled to the grid structures. The grid structure typically includes two sets of wire loops operatively oriented so as to form the horizontal (X coordinate) and vertical (Y coordinate) input lines of the grid for accurately determining the X-Y location of the stylus mechanism relative to the data entry surface.
The stylus mechanism is generally shaped in a pen-like or pointer-type of configuration. When the stylus is inductively coupled, it includes within the body thereof at least one electric coil, which coil may be inductively coupled to the grid by energizing either the coil or the grid with an A.C. voltage. Alternatively, the stylus may be conductively coupled to said grid structure and include contact means to a common or ground.
Each of the aforementioned wire loops which define the grid provides a discrete sensor for uniquely determining the X-Y location of the stylus on the data-input surface of the tablet. The X-Y location is determined by processing data signals generated when the stylus mechanism is brought into contact with the particular set of wire loops of the grid. However, in order to obtain the precise location of the stylus mechanism on said tablet surface, it is necessary to filter and demodulate the electrical signals generated by the wire loops. Further, the grid is subject to both inductive and capacitive interference by objects carrying an electric field (such as a radio or lighting). Thus, in order to achieve optimum accuracy in X-Y location determination, it is necessary to shield the grid from such externally introduced, performance degrading interference.
Prior art electronic digitizing tablets of the type described hereinabove rely upon coupling between said stylus mechanism and said electric grid in order to determine the X-Y location of information introduced onto the input surface thereof. Another, and more favored type of electronic digitizing tablet relies upon the interference created when a conductive stylus structure interacts with an electric field applied across the conductive, data input surface associated therewith. It is this electric field disturbance which is used to determine the X-Y location of information touch inputted onto, i.e. contacting, the input surface. While this field disturbing method has several advantages over the inductive contact method, it also suffers from different, but substantial limitations. Chief among these limitations is that the electric field established across the conductive, data input surface of the touch sensitive sensor is not perfectly linear, which non-linearity results in errors in the determination of the unique X-Y location (relative to the data input surface) of said inputted information. In the case of very low resolution touch sensitive sensors, minor field non-linearities can be tolerated; however, as should be apparent, as the resolution of the touch sensitive surface of said touch sensitive sensors increases (up to perhaps a resolution of 100 lines/in.), the greater the likelihood that even minor non-linearities in the electric field distribution will result in erroneous X-Y location determination. In order to compensate for errors which arise due to field non-linearity, the digitizing tablets of the prior art found that it was necessary to utilize special data processing techniques, which techniques added significant cost and complexity to the tablet.
In an effort to avoid the use of said special data processing techniques, the ohmic contact touch sensors of the prior art attempted to provide a more uniform or linear electric field distribution across the data input surfaces thereof. The field linearization was accomplished by employing special current distribution techniques. More particularly, the conventional manner of establishing a uniform electric field was to operatively dispose elongated current contacts along the boundaries of the conductive input surfaces of the sensors. However, such elongated current contacts were inherently maintained at an equipotential at all points along the length thereof. Since such contacts were equipotential, sensing the position of a touch input by detecting current differences existing at specific points along the length of the contact was impossible.
The prior art next attempted to provide a uniform electric field distribution across the conductive surface in a manner which would enable the accurate touch point location determination by forming a series of parallel rows of a plurality of short, conductive segments printed upon or overlaid onto said conductive input surface. The plurality of short segments not only provided for a more uniform field distribution, but allowed for those same segmented conductors to perform the position sensing function. An example of such a prior art system is U.S. Pat. No. 4,371,746 issued on Feb. 1, 1983 and entitled "Edge Terminations For Impedance Planes", the disclosure of which is incorporated herein by reference. Such segmented conductors did not provide perfect uniformity of the electric field and were expensive to implement. Note that the current distributors (segmented conductors) of such prior art systems were isotropic in current conduction behavior, i.e., the inherent electrical conductivity of said current distributors is substantially the same in all directions.
It should be noted that the necessity of providing a uniform and linear electric field distribution across the conductive surface of the sensor cannot be overemphasized. By way of example, assume that a potential of 10 volts is placed across that conductive surface (the segmented conductors at one boundary are at a +10 volt potential and the segmented conductors disposed at the opposite boundary are at a 0 volt potential). It is necessary that the potential taken at any point between those two sets of segmented conductors vary linearly. In this manner, a touch input half way therebetween would be a potential of 5 volts or a touch point three quarters of the way therebetween would be at a potential of 7.5 volts. Obviously, deviations from this linearity would adversely effect the accuracy of the determination of X-Y location.
One object of the present invention is to provide; (1) a more linear electric field distribution across the conductive surface of a touch sensitive position sensor; and (2) more accurate detection of information input upon said conductive surface. It is to be noted that anisotropic current distribution and collection is an important feature of the instant invention. Anisotropic current distribution, as used herein, will refer to the distribution of current across the conductive surface of a touch sensitive input sensor, which distribution is characterized by relatively good electrical conductivity, i.e. low impedance, in a first direction and relatively poor electrical conductivity, i.e. high impedance, in a second direction.
It is important to understand that the x and y coordinates of a touch point are substantially simultaneously sensed. During a given electronic scan cycle of the circuitry (of which there are typically 200 per second), one half of the cycle is dedicated to determining the x coordinate and the other half cycle is dedicated to locating the y coordinate. Specifically, during the first half cycle, i.e., from a first to a second clock pulse the electronic field is distributed in the y direction, by applying a forward biasing current to the current control means associated with the y field current distribution and collection means, and a reverse biasing current to the current control means associated with the x field current distribution and collection means. In this way the location of the touch point in the y plane may be located. During the second half cycle, i.e., from the second clock pulse to a third clock pulse, the current control means associated with the x field current distribution and collection means is forward biased while the current control means associated with the y field current distribution and collection means is reverse biased thus allowing the detection of the location of the touch point in the x plane.
With respect to the electronic copyboards, referred to hereinabove, visually detectable information may be manually entered onto the display surface (whiteboard surface), as by standard, dry erasable felt-tip markers. The visually detectable information is then digitized either by scrolling the visually detectable information past a stationary array of photosensitive elements or by passing an array of photosensitive elements mounted in a moveable arm over said information-bearing whiteboard surface. In either case, the visually detectable information cannot be digitized at the same time that it is being manually input onto said surface. Electronic whiteboards can thus be understood to utilize a two-step process in which information is first written thereupon which information can only be subsequently digitized.
More particularly, electronic imaging systems associated with whiteboards generally include either an array of photosensitive elements such as photosensors or an optical system with a charge coupled device, a data input surface upon which images or characters may be entered; a light source operatively disposed so as to illuminate the image-bearing surface being scanned, and means for effecting relative motion between the array of photosensitive elements and the image-bearing surface.
In operation, radiation provided by the light source is reflected from the image-bearing surface, the intensity of which reflected radiation varies depending upon the nature of the visual information disposed upon the surface. Dark portions of images on the surface will reflect less light than brighter portions; thus, images entered upon the surface as by a felt-tip marker, will reflect less light than areas of the surface not bearing an image. The photosensitive elements are then able to effect a detectable change in an electrical parameter, such as conductivity, in direct response to the amount a reflected light incident thereupon. This change e.g., in conductivity, is detected and relayed in the form of electrical signals for downstream processing. Said downstream processing is adapted to correlate the electrical signals received from the photosensitive array relative to the image-bearing surface. In this manner, the location and nature of the information on the information bearing surface of the whiteboard can be accurately displayed. Such electronic whiteboards are fully disclosed in commonly assigned U.S. patent application Ser. No. 885,907 filed July 15, 1986 and entitled: "Photosensitive Line Imager Utilizing A Moveable Scanning Arm" now U.S. Pat. No. 4,725,889, the disclosure of which is incorporated herein by reference.
It is noteworthy that all of these aforementioned electronic digitizing devices have gained some measure of commercial acceptance despite inherent technological limitations. The prior art electronic whiteboards all require cumbersome mechanical apparatus for effecting relative motion between the arrays of photosensitive elements and the image-bearing surface to be scanned. The mechanical apparatus for accomplishing relative motion are relatively complex and expensive, both in terms of the initial purchase price and the ongoing cost of servicing. Electronic digitizing devices of the prior art are further limited in their ability to digitize and reproduce color images, said devices often requiring multiple passes over the image-bearing surface relative to a plurality of arrays of photosensitive elements, each array sensitized to different portions of the electromagnetic spectrum. Finally, said prior art electronic digitizing devices are unable to instantaneously (i.e., in real time) digitize electronic signals from information entered upon the whiteboard surface; rather, the information can be digitized only after manual data input is completed and a scan cycle has been initiated. Thus, an unnecessary time delay is always present and the possibility exists if losing information due to inadvertent erasure from the image-bearing surface prior to the initiation of the reproduction process.
It is a further object of the present invention to provide a solid state copyboard having no moving parts and capable of the instantaneous display of visible information marked upon the surface thereof.